1. Field of the Invention
The present invention relates to recycling expanded polystyrene, especially to the recycling of flame retardant expanded polystyrene.
2. Description of the Prior Art
Expanded polystyrene (EPS) is in wide use in applications that require enhanced resistance to fire and heat. In these situations, which may account for some 50% of all EPS use, flame retardants are frequently added to the polystyrene resins to modify the combustibility properties of the plastic. The addition of flame retardant materials results in increased ignition temperatures, and a reduction in burning rates and flame spread.
Flame retardant modifiers used include halogenated hydrocarbons. Typically, the flame retardant used in polystyrene resins is a brominated hydrocarbon. Flame retardants that can be used in conjunction with polystyrene are listed in the Modern Plastics Encyclopedia, Vol. 56, No. 10A, 666-670 (Oct. 1979), hereby incorporated by reference. Some of the brominated hydrocarbon flame retardants that can be used with polystyrene include: brominated alicyclic; hexabromocyclododecane; octabromodiphenyl oxide; decabromodiphenyl oxide; brominated organic; and Trisdibromopropyl antimonite.
Besides having the positive, intended effect of modifying the combustibility properties of the polystyrene, the flame retardant also increases the melt flow rate of the polystyrene. Consequently, flame retardant polystyrene flows much more readily than does standard polystyrene. This makes it difficult, if not impossible, to recycle flame retardant polystyrene because processing equipment is typically set up for the melt flow rates of untreated polystyrene, not flame retardant polystyrene. In addition, the bromine in the flame retardant additive is corrosive to the molding equipment.
Melt flow rates of polystyrene can be measured using an extrusion plastometer. The extrusion plastometer generally used in the industry to measure melt flow rates of polystyrene is described in ASTM D 1238, hereby incorporated by reference. During normal operation, resin is melted in a cylinder of the instrument for a fixed period of time; a weighted piston is then permitted to push out the resin through an orifice of a specified length and diameter. Pieces of extruded resin are then cut off at timed intervals. Using the extruded specimen weight and the time to extrude, a flow rate is calculated. Flow rates are reported in g/10 min.
Flame retardant expanded polystyrene typically has a melt flow rates substantially above that of untreated expanded polystyrene, which is referred to throughout the specification as expanded polystyrene.
Adding further to the problem of recycling polystyrene is the fact that flame retardant and expanded polystyrene products are often co-mingled in waste streams. These products, however, cannot be separated by sight; thus, the rheological properties of the waste stream are inconsistent and depend on the percentage of flame retardant polystyrene contained in the waste stream. Recycling companies, as a result, have found it difficult and expensive to recycle expanded polystyrene products from a mixed waste stream and, for the most part, have simply not recycled these products.
To recycle expanded polystyrene, the polystyrene is first ground, usually to a fine particle size, and then washed or rinsed with water. After being rinsed, the ground polystyrene is condensed by heating it in an oven to a temperature between approximately 150.degree. C. and 200.degree. C. Following condensing, the polystyrene is extruded into long spaghetti like strands and then pelletized. The polystyrene is pelletized by chopping up the long strands of extruded polystyrene into approximately 1/16 inch pellets. Manufacturers can then use the pellets as feed stock for an extrusion or injection molding process to manufacture new products.
If flame retardant polystyrene is mixed in with the original recycled material, each of the resulting pellets can have different rheological properties depending upon the percentage of flame retardant polystyrene making up each individual pellet. Thus, manufacturers would have a difficult time controlling their injection molding and extrusion processes because they are set up to handle melt flow rates of untreated expanded polystyrene and are not accustomed to handling a feed with inconsistent rheological properties.
Another problem with polystyrene products is that they tend to collect static electricity. This typically limits their use as packaging materials for electronic components and devices that are sensitive to electrostatic discharge (ESD). When polystyrene materials are used, the electronic components are often shielded from electrostatic discharge in metallized bags or bags treated with an anti-static agent to prevent ESD damage.
At present, a need exists for a method of treating flame retardant polystyrene to reduce its melt flow rate so that it can be economically recycled. Also, an inexpensive anti-static agent that can be easily applied to polystyrene to dissipate and prevent static build up would be desirable.